Imagine a small tailpipe constantly pumping combustion byproducts into your engine’s crankcase. In effect, this is what is happening when your engine is running. Blow-by gases entering the crankcase by leaking past the pistons and rings during the combustion process need proper evacuation. If left unchecked, they cause numerous side effects, inducing engine problems that may seem unrelated.

Side effect #1: Crankcase pressure (“My engine leaks oil”)

The job of the Positive Crankcase Ventilation (PCV) system is to remove blow-by gasses from the crankcase by vacuum and recirculate them via the intake manifold to be burned in the engine. If the engine is producing blow-by gases faster than the PCV system can dispose of them, an increasing surplus becomes trapped in the crankcase, causing excess pressure and, inevitably, oil leaks. Even the most carefully sealed gaskets leak when confronted by rising internal crankcase pressure.

A properly functioning PCV system will expel the gases from the crankcase faster than the engine produces them. In addition, the low-level vacuum draws in fresh air to the crankcase from the crankcase breather. In 99% of normal driving conditions, this is how a properly functioning PCV system works. Obviously, the gasket’s job is made easier when the crankcase has low-level vacuum inducing the oil inward, rather than outward.

When the engine produces blow-by gases faster than the PCV system can manage, the increasing surplus finds its way out via the crankcase breather. In fact, if the system is working properly, the breather will almost always draw fresh air in, not push blow-by gases out. Also, these stray blow-by gases cause other nuisances.

A common sign of blow-by is evidenced by oil on the outer surfaces of the valve covers. This is often improperly remedied by wrapping a shop rag or sock around the base of the breather, which prevents a mess but doesn’t address the root of the problem. Of course, the smell from persistent oil leaks from the crankcase breather – oil residue that should be burned in engine – often permeates the car’s interior while driving. Excess oil in the base of the air cleaner is a further telltale sign of an engine with a closed PCV system (where the crankcase breather connects to the air cleaner base).

Side effect #3: Excessive blow-by going to the right place at the wrong time (“Why can’t I perfect my idle tuning?”)

Engines typically don’t produce much blowby at idle. Likewise, they cannot tolerate much crankcase airflow being recirculated into the intake at idle either, since the idle air-fuel mixture needs to be precisely controlled. For this reason, a properly functioning PCV valve restricts the amount of airflow in the intake stream under idle conditions. If the PCV valve flows excessive air at idle, the result can be unresponsive carburetor idle-mixture screw adjustment, poor idle quality, as well as EFI idle tuning difficulties.

How a Stock PCV valve works

Stock OEM-style PCV valves have remained unchanged for over 50 years. A typical stock-style PCV has one airflow channel; the air flow through this channel is controlled by a spring actuated piston. The flow rate at idle as well as the flow rate under cruising conditions and the vacuum level where the valve transitions between these modes are controlled by the spring’s stiffness and the piston’s geometry. These parameters are fixed and not adjustable.

Maintaining just the right amount of airflow through the PCV system is a critical component of any engine’s tune-up. Too much or too little airflow at the wrong times is detrimental; further, the ideal airflow profile can vary widely from one engine to another.

PCV Valves for Modified Engines

It has long been thought that it was adequate to use free-flowing valve cover breathers without a PCV valve to control blow-by on a street performance engine, but this is not true. Breathers alone will relieve some but not all crankcase pressure. Header e-vac systems and vacuum pumps are also an option but are generally not practical on street engines. The vacuum provided by a PCV system with its supply of fresh air that is drawn through the crankcase is the more effective method.

When speed parts such as an aftermarket camshaft and cylinder heads comprise an engine build, the PCV vacuum profile is altered as a result. Any performance engine build involves careful attention to all components selected, and a properly ventilated crankcase might be the last component considered, but it’s an important one. A properly functioning PCV system will cleanse the crankcase by circulating fresh air, collect harmful moisture and blow-by vapors and route these vapors back into the intake stream. This will not only help engine tuning and performance but also engine longevity.

Introducing Dual Flow PCV Technology

In 2016 M/E Wagner Performance was granted a patent for a novel PCV design which puts the user in control of all aspects of the PCV’s performance. This Dual Flow PCV is the first valve available specifically designed for street-driven performance engines, and it marks the first significant revision to the PCV valve’s design in over half a century.

Every Wagner dual-flow PCV valve is made in the USA and incorporates over 50 separate CNC-machining operations in its manufacture.

Dual Flow technology splits the airflow into two separate circuits for idle and cruising conditions. This allows the user to adjust the PCV system’s flow rate, as well as the vacuum level where the valve transitions from low to high flow. For low or fluctuating idle vacuum, Wagner’s dual-flow PCV can also be run in fixed-orifice mode, which maintains user adjustable airflow and full backfire protection. The valve’s check ball design provides superior backfire protection and is especially helpful in boost applications. All valves are 100% flow tested and retail for $129 with free shipping.

Another great technical article from MGI. The PCV system is unfortunately one of the most misunderstood and neglected items on nearly all engines. The detrimental effects of a missing or malfunctioning PCV system can’t be ignored. Moisture forms in the crankcase every time an engine goes through a heat/cool cycle. One of the by products of internal combustion engines using gasoline is carbon particles that enter the crankcase in small amounts along with the blow by.

Carbon is a fine particulate abrasive that mixes with the oil and heads straight to all bearings. A properly operating PCV system can help remove the blow by vapors before they have a chance to mix with the engine oil in sufficient amounts to become like liquid sandpaper.